Electroluminescence (EL) is an opto-electronic phenomenon in which a material emits light in response to an electric current passing therethrough, or to a strong electric field. This is distinct from light emission resulting from heat (incandescence), chemical reaction (chemiluminescence), sound (sonoluminescence), or other mechanical action (mechanoluminescence).
EL devices come in many forms. They can have a common flat capacitor structure (bidirectional current pathway), or a diode structure (unidirectional current path). EL devices with bidirectional current pathways, such as HFELs, include alternating current (AC) and direct current (DC) driven devices and can have a powder or thin-film electroluminescent (TFEL) material. Unidirectional EL devices include light emitting diodes which can comprise inorganic or organic materials. The electroluminescent material in organic light emitting diodes (OLEDs) can include a small molecule material or a polymer material. Depending on the driving mechanisms of the EL devices, the terminologies of active matrix EL (AMEL) or passive matrix EL (PMEL) can be used.
In EL devices, the EL material is sandwiched between two electrodes. One electrode can be transparent to allow the escape of light produced by the EL material. Glass coated with indium tin oxide (ITO) is commonly used as the front (transparent) electrode while the back electrode can a reflective metal or coated with a reflective metal.
When a voltage is applied to the electrodes, the positive and the negative electrodes can provide injection of holes and electrons, respectively, into the EL material. In the EL material, electrons and holes move towards each other in the applied electrical field and form excitons, which are bound excited states that can relax down into the ground state radiatively by emitting a photon. This process can be referred to as electroluminescence. EL devices are of interest in, for example, display, signage, and lighting.
OLEDs were first designed in the 1980s, see, e.g., C. W. Tang, S. A. Van Slyke, Organic Electroluminescent Diodes, Appl. Phys. Lett., 1987, 51, 913. More recent developments in OLED materials and applications are generally described in Kraft et al., Angew. Chem. Int. Ed., 1998, 37, 402-428, and Z., Li and H. Meng, Organic Light-Emitting Materials and Devices (Optical Science and Engineering Series), CRC Taylor & Francis (Sep. 12, 2006). The disclosures of these references are incorporated by reference in their entirety.
Oleg Vladimirovich Losev independently created the first LED in the mid 1920s and his research was distributed in Russian, German and British scientific journals. LEDs are fabricated from inorganic materials, such as indium, aluminum, gallium, arsenic and/or nitrogen. LEDs have been commercially available since the 1960s. LEDs are manufactured in small, discrete packages and are point light sources. More recent developments in LED materials and applications are generally described in E. Fred Schubert, Light Emitting Diodes, Cambridge University Press (Jun. 9, 2003). The disclosure of this reference is incorporated by reference in its entirety.
High field electroluminescence was first observed in silicon carbide (SiC) by Captain Henry Joseph Round in 1907. Round reported that a yellow light was produced when a current was passed through a silicon carbide detector. The first thin-film EL structures were fabricated in the late 1950s by Vlasenko and Popkov. These two scientists used a thin film of Zinc Sulfide doped with Manganese (ZnS:Mn). In 1974, Toshio Inoguchi and his colleagues at Sharp Corporation introduced an alternating current (AC) TFEL approach to EL displays at the annual meeting of the Society for Information Display. The Sharp device was a high-brightness, long-lifetime EL device which used zinc sulfide doped with manganese (ZnS:Mn) as the phosphor layer, and yttrium oxide (Y2O3) for the sandwiching insulators. More recent developments in electroluminescent materials and applications are generally described in Yoshimasa Ono, Electroluminescent Displays, World Scientific Publishing Company (June 1995), D. R. Vij, Handbook of Electroluminescent Materials, Taylor & Francis (February 2004), and Seizo Miyata, Organic Electroluminescent Materials and Devices, CRC (July 1997). The disclosures of these references are incorporated by reference in their entirety.